The present invention generally relates to an anti-lock brake controlling apparatus, and more particularly, to an anti-lock controlling apparatus, provided with a pedal stroke quantity detecting means for detecting a stepping quantity of a brake pedal which detects a road surface friction coefficient (road face .mu.), where the stepping quantity of a brake pedal detected by the pedal stroke quantity detecting means is one of several factors in a control operation.
An anti-lock brake controlling apparatus is adapted to detect a reduction in a wheel speed with respect to an estimated vehicle body speed, a skid situation of a wheel from speed changes of the wheel, and prevent locks by changing a wheel cylinder fluid pressure, thus shortening a braking distance, and ensuring stability and steerability.
The above described anti-lock controlling apparatus requires that friction coefficients on road surfaces on which a vehicle runs be determined correctly so as to effect a more suitable controlling operation.
It is preferable to consider the road surface friction coefficients to estimate the vehicle body speed, because the vehicle body speed is subject to influence from the road surface friction coefficients.
Generation of a lock symptom is also subject to influence from the road surface friction coefficients. On a high .mu. road surface where friction coefficients are high on the road surface (asphalt or the like), the lock symptoms are difficult to cause on wheels even if the wheel cylinder fluid pressure increase. On a low .mu. road surface where friction coefficients are low on the road surface (snow covered road, ice covered road or the like), lock symptoms occur even when the wheel cylinder fluid pressure is relatively low.
The road surface friction coefficients relate to a period of reduction time of a wheel cylinder fluid pressure in the anti-lock controlling operation. When the pressure of the wheel cylinder fluid is reduced by the switching operation of a so-called on/off type solenoid valve in the above described anti-lock brake controlling apparatus, a pressure reducing characteristic is in approximate exponential relation with respect to the opened time of the solenoid valve as shown in FIG. 28. In a case where the pressure is reduced from the P1,a of relatively high pressure, and in a case where the pressure is reduced from P2 (which corresponds to low .mu. road surface) lower in pressure than the P1 (which corresponds to high .mu. road surface), the reduction pressure width .DELTA.P1 is larger than the reduction pressure width .DELTA.P2. Therefore, a stop distance is longer because of excessive reduction pressure when the pressure reduction time in the anti-lock controlling operation has been set excessively, in spite of high .mu. road surface. If the pressure reducing time is set short in spite of the low .mu. road surface, the risk of a so-called cascade lock, where the four wheels of an automobile are directed at locking, is increased due to insufficient pressure reduction.
On the other hand, an anti-lock brake controlling apparatus for estimating the road surface .mu. from the changes in speed of the vehicle body, and an anti-lock brake controlling apparatus provided with a sensor for directly measuring the road surface .mu. are provided.
An anti-lock brake control apparatus of a type which estimates the road surface .mu. from the changing speeds of the above described vehicle body judges that the road surface which is low .mu. is a high .mu. road surface when the wheel speed of all four wheels once starts reducing at a time, and becomes insufficient in pressure reproduction, thus having a risk of further increasing a tendency of cascade lock.
A sensor for directly detecting the above described road surface .mu. is expensive, thus increasing a manufacturing cost.